Here is the nasal code to calculate the Kollsman shift.
# Typical usage: indicated_altitude = pressure_altitude -
kollsman(baro_setting)
k_ft = k_set = nil;
kollsman = func{
if (arg[0] == k_set) {return k_ft}
k_set = arg[0];
k_ft = 145442.156 * (1 - math.exp(math.ln(k_set/29.921260) * 0.1902632365))
}
1) This achieves the goal of realism in the sense that it allows
the autopilot code to calculate the Kollsman shift using only
information available to a real autopilot.
I'd be astonished if real-world autopilots used anything much
different from this.
2) This is computationally efficient in the overwhelmingly-likely
case that the baro_setting is not being changed very often.
3) If you want to standardize this across the FG fleet, put it in
some accessible place [perhaps atmo.nas] and let people call it
from there [as atmo.kollsman(...)] rather than cut-and-pasting
it in multiple places.
4) If you don't think this is -- for all practical purposes -- the
right answer, please explain what is the right answer ... and
explain how a pilot could tell the difference between this and
the right answer.
5) Since this has some advantages and AFAICT no disadvantages, it
removes any temptation to use the c++ altimetry object as an oracle
for computing the Kollsman shift.
===================================
Tangentially related note: I made one recent change to the package
of diffs:
http://www.av8n.com/fly/fgfs/atmo.diff
I rigged it up so that encoder.[ch]xx are no longer needed, and are
not even mentioned in the Makefile.am or anywhere else. When you
configure an <altimeter> you get an instance of the Altimeter class,
and when you configure an <encoder> you get a different instance of
the Altimeter class. The only difference is that the former has a
default <quantum> of zero, while the latter has a default <quantum>
of ten.
Users should not notice any difference (except that their altimetry
suddenly becomes much more accurate). The configuration files such
as generic-instrumentation.xml can stay exactly the same, and the
runtime interface (via the property tree) is upward-compatible.
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